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Astronomers from the University of Leicester and the CESR astrophysics lab in Toulouse reckon they've spotted an intermediate mass black hole - one of an elusive group of beasts weighing in between single star black holes and their supermassive cousins.

A team led by Sean Farrell was perusing X-ray sources dating back to 2000 when they spied an object in the galaxy ESO 243-49 emitting fluctuating X-rays "400 times as bright as the maximum value for a stellar black hole", as New Scientist explains.

Significantly, the X-ray emissions weren't from a galactic centre, where supermassive black holes commonly lurk, and the scientists ruled out a blazar galaxy - a type which also transmits fluctuating X-rays.

In this case, the "strong radio emissions" also associated with blazars were absent.

The astronomers also dismissed the possibility that the X-rays might come from a small black hole's jet which "could have made that object seem bigger if directly focused at Earth". The spectrum "did not suggest this", according to Farrell.

The team notes that "the existence of such intermediate-mass black holes is in dispute, and though many candidates have been proposed, none are widely accepted as definitive".

The team's claim for middleweight glory can be found in Nature (subscription required). ®